Expert ForumsMixing

Topic: Re: Is there a general formula to apply to a ribbon blender?

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What your are asking is whether there is a general formula to apply to a ribbon blenders. The right answer is that no absolute formulas exist, but rather the blenders follow some general relationships that work well for a range of powder properties. You actually should start with your housing dimensions, which do fit typical designs. First, the direction of the ribbon pitch for the outer ribbon is always toward the discharge. For a center discharge, the outer ribbons are divided at the middle of the mixer, with each half of the ribbons pushing toward the center discharge. The inner ribbons must push material in the opposite direction, toward the ends of the blender. All of the ribbons should be double-flight ribbons. Double flight means that each outer ribbon starts on one side of the center shaft and directly opposite that ribbon is another identical ribbon, both pushing the same direction. The two outer ribbons balance the loads on the blender shaft. The inner ribbons also are double flight for mechanical balance. In effect you have two outer and two inner ribbons on each end of the blender, with the ribbons in each end pushing in the opposite directions and each inner ribbon pushing in the opposite direction of the adjacent outer ribbon. Dimensions then become as much practical as they are a formula. With the housing being 2200 mm wide, the outer ribbons should be about 2000 mm in diameter, providing about a 5% clearance on each side of the blender. With 2000 mm diameter ribbons, their length will be half the length, or 2000 mm, which makes 2000mm a logical pitch for the ribbons. A 1:1 pitch to diameter is common. As with most helical ribbons, whether for powders or liquids, a ribbon width of 200 mm (10% of the diameter) is practical. Although not equivalent in material movement, the inner ribbon should be 200 mm to 300 mm wide. The inner ribbon is typically positioned about half way between the outer ribbon and the center shaft. The idea is to allow sufficient area between the ribbons for powder motion in opposite directions. The best technical book on mixing is the Handbook of Industrial Mixing, Wiley, 2004. However, that source is more about the functional design of mixers than the mechanical dimensions and design of mixing equipment. Equipment manufacturers provide little specific information about the design of their equipment. The answers by this expert are based on the best available interpretation of the information provided. The consequences of the application of this information are the responsibility of the user. If clarification is needed, please submit a further question.